The present invention relates to a process for producing a slurry suitable for the manufacture of a mixed fertilizer containing nitrogen plus phosphorus and possibly potassium.
The source of phosphorus commonly used in the manufacture of mixed fertilizers containing nitrogen, phosphorus, and potassium, i.e., so-called NPK fertilizers, is a crude phosphate, e.g., apatite. The aim is to convert the phosphorus present in apatite to such a form that it is available to plants. This occurs when nitric acid is caused to react with crude phosphate, whereby phosphorus is released as phosphoric acid. In the successive stages the produced acid mixture and the fed additional acids, phosphoric acid and possibly sulfuric acid, are neutralized to the desired degree by means of ammonia.
Apatite contains per one P.sub.2 O.sub.5 molecule an average of 3.35 CaO molecules, of which 3 are combined with P.sub.2 O.sub.5 and 0.35 with fluorine and other negative anions. Such a compound forms an apatite molecule, in which the P.sub.2 O.sub.5 is not available to plants. In the process the apatite molecule is dissociated by means of nitric acid, and thereby the P.sub.2 O.sub.5 is obtained in a soluble or ionizable form.
A slight excess of nitric acid is used in the reactions between apatite and nitric acid, and thereby a short reaction period and a high efficiency are achieved. With apatite, 6.8 mol nitric acid is used per one apatite molecule.
On ammonization of the mixture, ammonium nitrate is thus obtained, the P.sub.2 O.sub.5 changes into dicalcium phosphate (P.sub.2 O.sub.5, 2 CaO) through monocalcium phosphate (P.sub.2 O.sub.5, CaO), and part of the calcium remains in calcium nitrate Ca(NO.sub.3).sub.2.
Reaction between apatite and nitric acid: EQU P.sub.2 O.sub.5 .multidot. 3.35 CaO + 6.8 HNO.sub.3 .fwdarw. P.sub.2 O.sub.5 + 3.35 Ca(NO.sub.3).sub.2 + 0.1 HNO.sub.3
neutralization reaction: EQU P.sub.2 O.sub.5 + 3.35 Ca(NO.sub.3).sub.2 + 0.1 HNO.sub.3 + 4.1 NH.sub.3 .fwdarw. P.sub.2 O.sub.5 .multidot. 2CaO + 1.35 Ca(NO.sub.3).sub.2 + 4.1 NH.sub.4 NO.sub.3
the calcium nitrate, Ca(NO.sub.3).sub.2, must be removed since in a fertilizer it would cause insurmountable difficulties owing to its highly hygroscopic nature.
In the Odda process (Farm Chemicals, May 1967, p. 29) part of the calcium derived from the leaching of crude phosphate is removed by crystallizing it as calcium nitrate tetrahydrate. This process has, however, drawbacks, i.e., the expensive crystallization and the great number of byproducts which are difficult to market.
In another process, the PEC process (Farm Chemicals, May 1967 , p. 28 ), calcium is precipitated by means of some acid, e.g., phosphoric acid, in numerous ammonization tanks. A drawback of this process is the great number of ammonization tanks needed for raising the pH of the nitrophosphate slurry by degrees to a value suitable for the manufacture of mixed fertilizers.
Also known are different processes in which phosphoric acid is added together with nitric acid to the crude phosphate leaching stage, whereby phosphoric acid is used instead of part of the phosphorus in the crude phosphate. When nitric acid reacts with crude phosphate, phosphoric acid and calcium nitrate, among other things, are formed which, together with the phosphoric acid added, are thereafter neutralized with ammonia. These processes usually comprise several neutralization stages, or part of the neutralization is performed in a granulator, for the slurry greatly tends to thicken.